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← SRR-91
Surveillance radio system

SRS-91 was a complete wireless covert listening system (bug) operating on 350 MHz, developed in the early 1970s by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term development contract under the codename Easy Chair (EC). It uses the Dirty Pulse (DP) or '91' audio masking scheme that was developed exclusively for the CIA. Some parts, such as the SRT-93 transmitter, feature a 40 kHz subcarrier audio masking scheme.

SRS-91 system components
Low-power version of the SRT-91
Miniature 350 MHz transmitter (bug) with Dirty Pulse (DP) audio masking
Stereo variant of the SRT-91
Miniature 350 MHz transmitter (bug) with subcarrier (SC) audio masking
Modular surveillance receiver for RP and DP masked bugs
Improved modular surveillance receiver (vertical model)
Improved modular surveillance receiver (horizontal model)
Optional down-converter for the 1500 MHz band

  1. This transmitter was not developed by the NRP but by an unknown third party, probably in the US. It uses 40 kHz subcarrier audio masking and requires an SC-module in the SRR-91 or SRR-90 receiver.

Developement of the system was started around 1968, with the first prototypes using an early version of the DP audio masking scheme. After several improvements, further miniaturisation of the transmitters, and the move to a lower DC voltage, the first operational parts of the system were released in 1974, consisting of the SRT-91 transmitter and the SRR-91 surveillance receiver.

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The system introduced a new audio-masking scheme for pulse-modulated transmitters, that became known as Dirty Pulse (DP) or '91' masking. It works by converting analogue audio into a series of position-modulated pulses (PPM) of which the front edge is shifted in time under control of a noise generator, making it impossible to decode the audio in a non-compatible receiver.

The SRR-91 receiver had a novel modular design, that made it possible to quickly repair a faulty unit in the field, simply by swapping a couple of modules. Immediately after the release of the first units however, the CIA put in a request to study further improvements, such as different form factors and the possibility to add compatibility with subcarrier modulated transmitters [F].

The study resulted in the release in 1975, of two new modular receivers that were initially known as the SRR-91 Mark II, but were later renamed by the CIA to SRR-90A and SRR-90B. Being fully upgradable, they would become the standard CIA receivers for the next two decades for the majority of CIA bugs. Also in 1975, the SRT-90 was introduced, which was in fact a low-power variant of the SRT-91. It was housed in a nearly identical enclosure, but was about 1 cm shorter.

Other surveillance systems
  1. Ontwikkeling (development) SRS-91
    NRP (notes), August 1968 - January 1971. CM302629/A.

  2. Manual for SRS-91 Engineering Model
    NRP, September 1968. CM302629/B.

  3. Operating Manual for SRS-91 Equipment
    NRP, January 1970. CM302629/C.

  4. Technical Manual for SRS-91 Equipment 1
    NRP, January 1970. CM302629/D.

  5. Technical Manual for SRS-91A (prototype, draft)
    NRP. March 1973. CM302629/E.

  6. Study of Further SRS-91 System Developments
    NRP, October 1973. CM302629/F.

  7. Operating and Test Manual for SRS 90-91 Dual-Channel Equipment (prototype)
    NRP, March 1976. CM302629/G.
  1. To be used with SRS-52 or SRS-56 Technical Manual.

  1. NRP/CIA, Collection of documents related to SRS-91
    Crypto Museum Archive, CM302629 (see above).
Further information
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Crypto Museum. Created: Saturday 06 May 2017. Last changed: Sunday, 25 December 2022 - 12:08 CET.
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